How do I change my @username?

In order to change your username, you need to do so on Twitter itself. You can find the link to do so here:

http://support.twitter.com/articles/14609-how-to-change-your-username

Once you change your username on Twitter, you can log out of TweetCaster (tap the four squares in the upper left, long-press your avatar and select sign out, then sign back in) and log back in and the app will reflect your new username.